docs(pinia-orm): MorphToMany example Taggable is missing primaryKey (#1959)

Author: CodeDredd

docs/content/1.guide/3.relationships/5.polymorphic.md

@@ -231,6 +231,8 @@ class Tag extends Model {
 class Taggable extends Model {
   static entity = 'taggables'
 
+  static primaryKey = ['tag_id', 'taggable_id', 'taggable_type']
+
   static fields () {
     return {
       id: this.attr(null),